1. Field of the Invention
The present invention is directed to an antenna, especially of superconductive material, for medical applications in nuclear magnetic resonance tomography.
2. Description of the Prior Art
In nuclear magnetic resonance tomography utilized in medical diagnostics, atomic nuclei with a magnetic moment in the examination subject are excited into higher-energy conditions in the human body by pulsed magnetic fields, these conditions being defined by a static magnetic field. Relaxation processes determine the return of the nuclei into the low-energy condition. Radio-frequency signals are thereby emitted that are measured, interpreted and displayed in the form of tomograms. Specific radio-frequency antennas operated in the near field are required for this purpose that can pick up the very low levels of the emitted nuclear signals with a high signal-to-noise ratio.
The amplitude of the radio-frequency signal of the relaxing spins is proportional to the Larmor frequency, to the transverse radio-frequency component of the antenna relative to the static magnetic field and to the local magnetization in the volume element in which the emitted signal originates. In identifying the location-dependency of this amplitude it is not possible to distinguish whether, for example, the local magnetization is responsible for a specific signal quantity or whether the transverse radio-frequency component is responsible for a specific signal quantity. A drastic falsification of the image content can thereby arise. An antenna employed in nuclear magnetic resonance tomography should therefore supply an optimally location-independent amplitude in the entire volume under investigation. Since the magnetization as well as the Larmor frequency increase proportionally with the static magnetic field, the amplitude of the signal detectable by the antenna increases approximately quadratically with the frequency, whereas the noise simultaneously emitted by the body increases only linearly with the frequency. In addition to repeated measurements with data acquisition and switching from linear to circular polarization, an enhancement of the signal-to-noise ratio is mainly achieved by a boost of the frequency, or of the static magnetic field connected therewith. The upper limit of the usable range of field strength is established by the already highly pronounced standing wave and resonance effects in the conductive body tissue that influence imaging of structure lying deep under the skin. Standard systems therefore operate in a static field strength range from 1 through 4 T. Complicated superconductive coils are required in order to achieve these high magnetic field strengths. Systems with normally (room temperature) conductive magnetic field coils or permanent magnets can only manage to produce a static field with a field strength in a range from 0.2 through 0.5 T. Above all, the low signal-to-noise ratio limits the resolution of the system in this low-frequency range.
When only a relatively small subject region is to be imaged, then the noise emitted by the body can be reduced by making the antenna radius smaller. Such small radius antennas are referred to as surface coils or local coils. Annulus antennas are mainly used for this purpose. The limit of miniaturization is established by the set noise of the antenna and the reduction in size of the imaging volume. Such surface coils generally exhibit relatively poor homogeneity, and in order to avoid the inhomogeneity from making a significant (detrimental) contribution to the image result, the far more homogeneous whole-body coil is used for transmission. The resonant frequency of the surface coil must then be detuned during the transmission phase so that the resonant frequency of the whole-body antenna is not varied and so that the possibility of focussing of radio-frequency power onto the organ under examination due to resonant coupling with the surface antenna is suppressed in all cases. Very complicated detuning circuits thereby become necessary for safety reasons.